Instead of using superposed subtractive pictures it is possible to make use of the additive superposition of the pictures, by means of an instrument called the photochromoscope or chromoscope.
Then, instead of each individual constituent positive being colored in the subtractive or complementary color, ordinary black and white transparencies are used, which are illuminated by the three fundamental colors and are optically superposed, giving the impression of a complete colored picture. The results obtained by this method are extremely pleasing, especially when seen stereoscopically, but, as in the stereoscope, only one person can examine the results at one time.
The following instructions should enable anyone to make one of these instruments; these have been taken from Konig's work, as they are the only working instructions that have been published. The instrument is shown in section and plan in the accompanying diagrams. Fig. 15 shows a section of the chromoscope; the box is made with steps with apertures at L, L', L for the light filters and transparencies. M and M' are two transparent mirrors at an angle of 45 degrees to the bottom of the box. R is a mirror or white card or preferably a sheet of opal glass, which is hinged so as to allow of adjustment to obtain the best illumination of the aperture L". The transparencies are placed on top of the filters. The eye sees through O the transparency placed at L" through the mirrors M and M'. The picture at V is reflected by the mirror M' and passes through M to the eye. The picture at L is reflected by the mirror M to the eye at 0. The last two pictures are thus seen in an inverted and upright position at L" and combine with the picture which actually is at this plane. The measurements are so adjusted that the three pictures fuse into one.
As the image of a mirrored object appears to be as far behind a mirror as the object itself is in front of the mirror, LM must be equal to ML", and L'M' equal to M'L". Therefore, LM - L'M' must equal ML" - M'L", or the distance of the mirrors M and M' from one another must be equal to the distance of L from L'.
The wood may be any well-dried kind, about 1 cm thick, which will not warp. For 9x12 cm pictures, or practically, with suitable-sized apertures, for 3 1/4 X 4 1/4 or 4x5 plates, the outside measurements should be about as follows:
The horizontal surfaces AGKF and HJEM have an aperture of about 10 x 13 cm exactly in the middle; the vertical surface CDEJ also has a similar size aperture, the upper edge of which is exactly as far from JE as the nearest edge of the aperture HJEM is from JE. One side wall carries a flap which is provided with hinges and turn-buckle, which is just large enough for the reflector R with the base that carries it to be stored away inside the box.
On a board of exactly the size of the inner bottom surface of the box the two mirrors M and M' (Fig. 15) are fastened. These mirrors should measure about 135 x 135 mm, and are placed with their lower edges screwed on two metal angle plates which should be screwed to the board. The shorter side of these plates should slope about 5 mm above the board. The longer side should have a slit about 25 mm long, through which passes the binding-screw. This slit enables one to shift the position of the mirrors slightly. The mirrors are supported behind by two screws C and C', by means of which the angle of the mirrors can be slightly altered. These screws should be perpendicular to the surface of the mirrors, that is, at an angle of 45 degrees to the bottom of the box. In order to support the mirrors they may have on their upper edges a narrow metal strip, to which may be soldered a hard brass or steel spring, which may be hooked to the baseboard, or to little blocks of wood projecting from the sides of the box.
The distance of the two mirrors from one another is the same as GH (Fig. 16), that is, 125 mm. The rear mirror should almost touch the back wall CDEJ at the top. The transparency, placed at L" (Fig. 15), should be illuminated by the mirror or reflector R, which should be adjustable, so as to enable the best illumination to be obtained. The whole apparatus should be fastened to a stout baseboard, so that it may be directed to the sky or any illuminant that is used.
An aperture should be cut in the front wall ABF, like the lens-board of a camera front, and the piece again fitted in with screws and turn-buckles. In this cut-out piece should be fastened an ordinary bi-convex or reading glass of from 35 to 40 cm focus, so that its center or optical axis should be exactly level with the middle of the aperture CDEJ. This lens need not be achromatic; one of the ordinary reading glasses is quite satisfactory. As large a lens as possible should be chosen, so that both eyes may be used; and the lens should be blocked out with black paper or a metal frame so that only a horizontal slit remains for the eyes to look through. The inside of the apparatus should be painted dead black, so that no white light is reflected to the eye. An ordinary shellac varnish, thinned with alcohol, and mixed with lampblack, so as to give a dead surface, may be used, or the wood may be chemically stained.
As pointed out on page 24 we must, as this is an additive process, use the fundamental colors for the filters, which are placed in the three apertures L, L', L", and which illuminate the three transparencies. The latter must be placed on filters of the same color as was used to make the negatives, that is, the transparency from the negative taken through the red filter must be illuminated by red light, the green transparency by green and the blue by blue-violet light. The filters for this purpose may be those suggested on pages 171,173 for screen-plate printing, or may be made from the following stock solutions of dyes. As these filters are merely used as light absorbers, or screens, and are not required to give accurate images, there is no need to be so careful in their preparation as with taking filters, that is to say, they need not be cemented nor need the glass be optically so perfect as in the latter case; old negative glasses, free from bubbles and scratches, may be used with satisfactory results. But it is important that the coatings should be as even as possible. Two colored screens should be placed film to film for each color, as it is thus possible to obtain more even results.